Copper foil with low profile bond enhancement

ABSTRACT

A composite material, comprising a carrier strip the carrier strip comprising a first side the first side comprising a substantially uniform roughness, an electrolytically deposited copper foil layer having opposing first and second sides and a thickness of from 0.1 micron to 15 microns and the entire metal foil layer thickness having been deposited from a copper containing alkaline electrolyte, and a release layer effective to facilitate separation of the metal foil layer from the carrier strip disposed between and contacting both the first side of the carrier strip and the second side of the metal foil layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This Application is a Continuation-in-part application of U.S. Ser. No.09/784,547 filed Feb. 15, 2001, now U.S. Pat. No. 6,569,543.

This patent application relates to U.S. Ser. No. 09/522,544 filed Mar.10, 2000, now U.S. Pat. No. 6,346,335. The disclosures of U.S. Ser. No.09/784,547 and U.S. Ser. No. 09/522,544 are incorporated herein byreference in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a composite material having an interveningrelease layer. More particularly, a copper foil layer is releasablybonded to a carrier layer for transport and assembly. The release layerdisposed between the carrier layer and the copper foil layer facilitatesseparation. The copper foil layer may be laminated to a dielectricsubstrate in the manufacture of printed circuit boards. A low heightprofile bond enhancing layer is formed on a surface of the copper foillayer opposite the release layer.

2. Description of Related Art

As electronic devices evolve, there is a need for thinner and smallerprinted circuits. This decrease in size leads to a requirement for finerline spacing to increase circuit trace density.

Most printed circuit boards have a dielectric substrate, such as anepoxy or polyimide, laminated to a layer of copper foil. The copper foilis etched into a desired circuit pattern. As the need for finer lineresolution increases, thinner copper foil is required. This is becausewhen copper foil is etched, etching occurs in both a vertical directionand in a horizontal direction at about the same rate. While the verticaletching is required to create spaces between adjacent circuit traces forelectrical isolation, horizontal etching at the sides of a trace damagesthe integrity of the circuit traces. Horizontal etching limits theminimum line-to-line spacing to approximately twice the thickness of thecopper foil. Another problem with thicker copper foil is that a longertime is required to etch the foil increasing the manufacturing cost andincreasing the environmental concern due to the disposal or reclamationof dissolved copper. Yet another problem arises from the presence ofNi/Au overhang. Ni/Au overhang occurs when a Ni/Au coating is applied asan etch resist and remains on the finished product to preservesolderability. When a thick copper foil coated with with Ni/Au isetched, the etch undercut can be severe resulting in breakage of theNi/Au overhang and possibly causing a short circuit in the finishedproduct.

One copper foil presently utilized in the manufacture of printed circuitboards is referred to as one-half ounce foil. One square foot of thisfoil weighs approximately 0.5 ounce and has a nominal thickness of about18 microns. Thinner copper foil, such as 9 micron thick foil, isavailable in the marketplace, however special care is required inhandling 9 micron foil to prevent wrinkling and damage.

Facilitating the handling of 9 micron, and thinner, foils is the use ofa carrier strip. The carrier strip is releasably bonded to the foil formanufacturing and lamination. Once the foil is laminated and supportedby a dielectric, the carrier strip is removed. One common carrier stripis aluminum that may be removed by chemical etching, such as byimmersion in sodium hydroxide, without damage to the copper foil.Etching is time-consuming and disposal may create environmentalproblems.

Alternatively, a carrier layer, typically formed from copper, is coatedwith a release layer. The copper foil layer is formed on the releaselayer, typically by electrolytic deposition. Adhesion between therelease layer and the copper foil layer is high enough so that thecopper foil layer does not separate from the carrier layer prematurely,but is also sufficiently low that separation of the carrier layerfollowing lamination does not tear or otherwise damage the copper foillayer.

U.S. Pat. No. 3,998,601 to Yates et al. discloses a release layer formedfrom sulphides, chromates and oxides. An alternative release layer isdisclosed to be chromium metal. U.S. Pat. No. 4,503,112 to Konicekdiscloses that chromium metal release layers have unpredictable adhesionand that preferred release layers include nickel, nickel/tin alloys,nickel/iron alloys, lead and tin/lead alloys. U.S. Pat. No. 5,114,543 toKajiwara et al. discloses a composite release layer having an immersiondeposited chromate layer that is coated with an electrolyticallydeposited copper/nickel alloy. The U.S. Pat. Nos. 3,998,601; 4,503,112and 5,114,543 patents are incorporated by reference herein in theirentireties.

U.S. Pat. No. 5,066,366 to Lin discloses forming a release layer on acopper alloy foil carrier by treating the carrier with an aqueoussolution containing chromic acid and phosphoric acid. While a generallyacceptable process, areas of unacceptable high adhesion may occur when achrome phosphate release layer is formed directly on a copper alloycarrier. U.S. Pat. No. 5,066,366 is incorporated by reference in itsentirety herein.

There remains a need for an improved release layer that consistentlyprovides adequate adhesion between a carrier layer and a copper foillayer to insure that the copper foil layer remains attached to thecarrier layer during transport and processing, such as lamination to adielectric substrate. However, the adhesion to the release layer issufficiently low that the carrier layer may be removed followinglamination without damaging the copper foil layer.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide a thin metallicfoil that is releasably attached to a carrier layer. A second object ofthe invention is to provide a method for the manufacture of the metallicfoil/carrier layer composite. A further object of the invention is toprovide a thin copper foil useful for lamination to a dielectricsubstrate for the manufacture of printed circuit boards and flexiblecircuits.

It is a feature of the invention that the metal foil is releasablyattached to a carrier layer and a force of at least 0.02 pound per inchis required to separate the layers thereby insuring that the metal foillayer is not prematurely released. It is a further feature of theinvention that a maximum force of 2 pounds per inch, and typically lessthan 1 pound per inch, is required to separate the metal foil layer fromthe carrier layer thereby facilitating removal of the carrier layerwithout damage to the copper foil layer.

A further feature of the invention is that the chemical solutionsutilized for deposition of the release layer are dilute aqueoussolutions that are believed to present less of an environmental hazardthan more concentrated electrolytes previously utilized to depositrelease layers such as metallic chromium.

Among the advantages of the invention are that the metal layer may be athin copper foil with a thickness of 15 microns or less. Such a thinfoil facilitates the manufacture of printed circuit boards and flexiblecircuits with fine features. A further advantage is that the carrierlayer is mechanically separable from the metal foil layer and does notrequire etching for removal.

A further advantage is that the foils of the invention have less surfaceroughness than conventionally formed foils. As a result, undercuttingduring etching is reduced. Furthermore, the low-profile treated surfaceis more suitable for high impedance/high frequency applications. Thesmoothness of the surface of the copper foil opposite the release layerallows for improved imaging and fine line capability in themanufacturing of circuits.

In accordance with the invention, there is provided a compositematerial. The composite material has a support layer and a metal foillayer. A release layer is disposed between and contacts both the supportlayer and the metal foil layer. This release layer consists essentiallyof an admixture of a metal and a non-metal.

In one embodiment of the invention, the composite material is thenlaminated directly to a dielectric substrate.

There is further provided a method for the manufacture of a compositematerial that includes the steps of (1) providing an electricallyconductive support layer; (2) anodically treating the electricallyconductive support layer in a first aqueous electrolyte that containsfirst metal ions and hydroxide ions; (3) subsequently cathodicallydepositing a release layer onto the electrically conductive supportlayer in a second aqueous electrolyte that contains second metal ionsand hydroxide ions; and (4) electrolytically depositing a metal foil onthe release layer.

One embodiment of this method of manufacture includes the additionalsteps of laminating the metal foil layer to a dielectric substrate andthen separating the electrically conductive support layer from thelaminate at the release layer. The metal foil layer, now bonded to thedielectric layer, may then be formed into a plurality of electricallyisolated circuit traces.

The above stated objects, features and advantages will become moreapparent from the specification and drawings that follow.

IN THE DRAWINGS

FIG. 1 illustrates in cross-sectional representation a compositematerial in accordance with the invention.

FIG. 2 illustrates in cross-sectional representation the compositematerial of the invention laminated to a rigid dielectric as a precursorto a printed circuit board.

FIG. 3 illustrates in cross-sectional representation the compositematerial of the invention laminated to a flexible dielectric as aprecursor to a flexible circuit.

FIG. 4 is a perspective view of the printed circuit board precursorsubsequent to removal of the carrier layer.

FIG. 5 illustrates in top planar view circuitry formed from thestructure of FIG. 4.

FIG. 6 illustrates an alternative release layer in cross-sectionalrepresentation.

FIG. 7 is a photomicrograph at a magnification of 5,000× illustrating avery low surface profile bond strength enhancement in accordance withthe invention.

FIG. 8 is a cross-sectional view of the very low surface profile bondstrength enhancement of FIG. 7 embedded in a dielectric substrate.

FIG. 9 is a photomicrograph at a magnification of 5,000× illustrating atypical low surface profile bond strength enhancement as known from theprior art.

FIG. 10 is a cross-sectional view of the low surface profile bondstrength enhancement of FIG. 9 embedded in a dielectric substrate.

FIG. 11 is a photomicrograph at 450× illustrating the surface topographyof a dielectric layer upon removal of a very low surface profile bondstrength enhancement in accordance with the invention.

FIG. 12 is a photomicrograph at 450× illustrating the surface topographyof a dielectric layer upon removal of a low surface profile bondstrength enhancement as known from the prior art.

FIG. 13 is a photomicrograph at 450× of the surface topography of adielectric layer following removal of a low surface profile bondstrength enhancement illustrating incomplete copper removal.

FIG. 14 graphically illustrates release layer separation force as afunction of lamination temperature.

FIG. 15 illustrates in cross-sectional representation the compositematerial of the present invention with an interposed dark layer.

DETAILED DESCRIPTION

FIG. 1 illustrates in cross sectional representation a compositematerial 10 in accordance with the invention. The composite material 10includes a support layer 12 and a metal foil layer 14. The support layer12 may be formed from any material capable of supporting the metal layer14. Preferably, the support layer 12 is formed from an electricallyconductive metal and has a thickness of at least 18 microns (1micron=1×10⁻⁶ meter). Suitable materials for the support layer includestainless steel, aluminum, aluminum alloys, nickel, copper and copperalloys.

When the support layer is stainless steel, a release layer, as describedbelow, may be optional.

Preferred for the support layer are copper alloys such as those alloysdesignated by the CDA (Copper Development Association, New York, N.Y.)as copper alloy C110 (nominal composition by weight 99.95% copper(minimum) and 0.04% oxygen), copper alloy C715 (nominal composition byweight of 30% nickel and 70% copper), copper alloy C510 (nominalcomposition by weight of 94.5% copper, 5% tin and 0.2% phosphorous) andcopper alloy C102 (oxygen-free high copper having a minimum coppercontent of, by weight, 99.9%) as well as brasses, mixtures of copper andzinc containing up to 40%, by weight, of zinc.

Most preferably, the support layer 12 is a wrought material as opposedto electrolytically formed. The wrought materials tend to have a higherstrength and a higher stiffness facilitating handling of the materialsand enhancing peelability of a deposited foil. The support layer may becoated with a copper or nickel flash to cover up defects, such asincurred during rolling, that may interfere with the deposition orremoval of the foil layer.

The support layer 12 may be a single material or composite material withthe second layer applied by any known process including rolling, platingand sputtering. Combinations of copper and nickel or copper and aluminumare believed useful as composite support layers. In one embodiment, thesupport layer 12 is comprised of a copper-nickel-silicon base alloy.

Preferably, the thickness of the support layer 12 is from 18 microns to140 microns and more preferably from 35 microns to 70 microns.

The metal foil layer 14 is any electrolytically deposited metal or metalalloy and is preferably copper. The metal foil layer typically has athickness of under 15 microns and more preferably is under 10 microns.In accordance with one aspect of the invention, most preferably, themetal foil layer is electrolytically deposited from an alkaline coppercontaining electrolyte followed by electrolytic deposition from anacidic copper containing electrolyte and has a thickness of from about 1to about 6 microns and nominally about 5 microns. In accordance withanother aspect of the invention, the metal foil layer iselectrolytically deposited only from an alkaline copper containingelectrolyte and has a thickness of from about 0.1 micron to 6 micronsand nominally about 5 microns. As described below, the metal foil layer14 may be deposited from a single electrolyte or from combinations ofmultiple electrolytes.

Disposed between and contacting both the support layer 12 and the metalfoil layer 14 is a release layer 16. The release layer 16 consistsessentially of an admixture of a metal and a non-metal, with the bulkbelieved to be the non-metal. It is believed that the metal component ofthe release layer constitutes from 5% to 40%, by atomic percent.

Suitable metals are those that are reversibly, electrochemically,oxidizable in a suitable electrolyte, as opposed to dissolving. The listof suitable metals includes nickel, chromium, titanium, copper,manganese, iron, cobalt, tungsten, molybdenum and tantalum.

Preferred metals are nickel, chromium and mixtures thereof. Preferrednon-metals are oxides, hydroxides, phosphates and chromates of themetals. Preferred combinations are mixtures of chromium and chromiumoxide, nickel and nickel oxide, chromium and chromium phosphate, nickeland nickel chromate, and nickel and nickel phosphate. The release layeris quite thin, on the order of 0.012 micron (120 angstroms) thick andtypically from about 0.001 micron to about 0.05 micron thick.

A most preferred release layer consists essentially of chromium and atleast one non-metal selected from the group consisting of oxides ofchromium and hydroxides of chromium. It has been established that theforce required to separate the metal foil from the carrier strip withthis most preferred release layer is consistently less than 7.1 kg/m(0.4 pounds per inch) even after exposure to temperatures of up to 380°C. for one hour. Since the metal foil is typically laminated, using heatand pressure, to a dielectric substrate prior to separation from thesupport layer, the release force as a function of temperature is animportant consideration.

Alternatively, the release layer 16 is a composite as illustrated incross sectional representation in FIG. 6. A first portion 30 of therelease layer 16 is a metallic layer, as described above, and ispreferably selected to be nickel, chromium or a mixture thereof. Thisfirst portion 30 directly contacts the support layer 12 and is typicallydeposited by electroplating. Other methods of deposition such asimmersion plating or vapor deposition may also be utilized.

A second portion 32 of the release layer 16 is an admixture of a metaland a non-metal as described above. The second portion 32 directlycontacts the metal foil layer 14.

With reference back to FIG. 1, on a side 18 of metal foil layer 14opposite the release layer 16, a bond strength enhancing agent 20 may bedeposited. Suitable bond strength enhancing agents includeelectrolytically deposited copper dendrites or copper-nickel dendriteshaving a height of between about 0.5 and 2.7 microns and a height todiameter aspect ratio of between about 1 and 5. Such dendrites may beelectrolytically deposited from an aqueous solution containing copperions utilizing copper or lead electrodes with the composite material 10as the cathode. Pulses of DC current are applied between the anode andthe cathode as more fully described in U.S. Pat. No. 4,515,671 to Polan,et al., that is incorporated by reference in its entirety herein Otherbond strength enhancing agents include an electrolytically depositedmixture of chromium and zinc as disclosed in U.S. Pat. No. 5,230,932 toLin, et al., a silane based coating as disclosed in U.S. Pat. No.5,071,520 to Lin, et al., copper oxides, mechanical abrasion ofsurfaces, alternating current etching and micro-etching.

Bond strength enhancing processes that give a very low surface profile,as achieved with the pulsed DC current of U.S. Pat. No. 4,515,671, arepreferred. Preferably, the average surface roughness (R_(a)) is 0.40micron or less. The average surface roughness is defined as thearithmetic average value of all absolute distances of the roughnessprofile from the center line within the measuring length. The roughnessprofile is determined utilizing a profilometer with a diamond stylus(contact method).

Typical low profile surface enhancements have a nodule height of greaterthan 3 microns and a nominal R_(a) value in excess of 0.4 micron. Thevery low profile surface enhancement of the invention has surfaceenhancements with a nodule height of between 0.5 micron and 2.7 microns.Preferably, the maximum nodule height is between 1.8 microns and 2.5microns. The R_(a) value is less than 0.4 micron and preferably R_(a) isbetween 0.20 and 0.35 micron.

The desirability of a lower surface profile is contrary to the typicalsurface profile applied to relatively thick, on the order of 12 micron,foil. Typically, higher surface profiles are utilized to maximize peelstrength once the copper foil is laminated to a dielectric circuitboard.

FIG. 7 is a photomicrograph at 5,000 times magnification illustratingthe very low surface profile bond strength enhancement of the invention.FIG. 8 is a cross-sectional view at a magnification of 450 times thatshows the bond strength enhancement 20 embedded into dielectricsubstrate 22.

FIG. 9 is a photomicrograph at 5,000 times magnification illustrating alow surface profile bond strength enhancing agent as known from theprior art. FIG. 10 is a cross-sectional view at a magnification of 450times that shows the bond strength enhancing agent 20 embedded intodielectric substrate 22.

As shown in FIGS. 11 and 12, when circuit traces 26 are formed byetching the metal foil layer, the exposed surface of dielectricsubstrate 22 has a topography that replicates the texture of the bondsurface enhancing agent. FIG. 11 is a photomicrograph of the surfacetexture resulting from etching a copper foil with the very low surfaceprofile bond enhancing agent of the invention. The magnification is 450times.

FIG. 12 is a photomicrograph at a magnification of 450 timesillustrating the surface of the dielectric substrate 22 followingremoval by etching of a copper foil with a typical low surface profilebond enhancement agent as known from the related art. A disadvantage ofthe typical low surface profile bond strength enhancing agent is moreapparent with reference to FIG. 13 that is also at a magnification of450 times. FIG. 13 illustrates that with the typical low surface profilebond strength enhancing agent, metallic dendrites may be embedded deeplyinto the dielectric substrate 22 and etching does not effectively removeall of the copper between adjacent circuit traces 26 leading to a shortcircuit 34.

Further, the smoother etch-exposed dielectric surface mirroring the verylow profile bond surface enhancement (FIG. 11) is much less likely toentrap contaminants and is much easier to clean as compared to the moretextured etch-exposed dielectric surface mirroring the typical lowprofile bond surface enhancement (FIG. 12). Therefore, it should bepossible to achieve a higher surface insulation resistance (SIR)capability. A high SIR is important in the design of high densityinterconnect circuits where the circuit traces are routed closetogether, for example, 25-125 micron circuit traces separated by 25-100micron spaces.

For the manufacture of a printed circuit board, the composite material10 is bonded to a dielectric substrate 22 forming circuit precursor 36as illustrated in FIG. 2. Metal foil layer 14 may be laminated throughthe addition of heat and pressure to a rigid dielectric for themanufacture of a printed circuit board. Typical lamination parametersare a temperature of about 180° C. for 50 minutes. Optionally, a polymeradhesive may assist in formation of the bond. Typical rigid materialsfor the dielectric substrate include fiberglass reinforced epoxies, suchas FR4. The dielectric substrate may also be an electrically conductivematerial coated with a dielectric material such as a metal cored printedcircuit board substrate or anodized aluminum.

Alternatively, as illustrated in FIG. 3, the dielectric substrate 22 isa flexible polymer film such as a polyimide or polyamide. In thisinstance, the use of a polymer bond agent 24 such as acrylic or epoxypolymer is preferred. As in the preceding embodiment, metal foil layer14 is bonded to the dielectric substrate 22 forming circuit precursor36. Rather than laminating the flexible polymer to the metal foil layer,the flexible polymer may be cast on to the metal foil layer as a liquidor gel and cured to a flexible film.

After the composite material 10 is bonded to the dielectric substrate22, the carrier layer 12 and release layer 16 are removed by mechanicalmeans. Typically, removal is by applying a force to the carrierlayer/release layer in one direction and an opposing force to thedielectric substrate/metal foil layer in a 90° direction. The forces maybe either manually or mechanically applied. The force required forseparation, referred to as release force, is at least 0.02 pound perinch and preferably at least 0.05 pound per inch. This minimum releaseforce is required to prevent the metal foil layer 14 from separatingfrom the support layer 12 prematurely, such as during transport orduring bonding to the dielectric substrate. The release force shouldalso be less than 2 pounds per inch and preferably less than 1 pound perinch to ensure that during removal the metal foil layer remains adheredto the dielectric substrate 22 and does not tear or partially remainwith composite material 10. Preferably, the release force is between0.02 pound per inch and 2.0 pounds per inch and more preferably betweenabout 0.05 pound per inch and 1.0 pound per inch.

FIG. 4 is perspective view of a circuit precursor 36 with metal foillayer 14 bonded to dielectric substrate 22. While FIG. 4 shows a singlemetal foil layer bonded to the dielectric substrate 22, additional metalfoil layers may be bonded to top surface 25 of the metal foil layer toform a multi-layer printed circuit board.

With reference to FIG. 5, the metal foil layer 14 may be chemicallyetched to form a printed circuit panel 44 having a plurality ofconductive features such as circuit traces 26 and die pads 28.Electrical isolation between conductive features is provided bydielectric substrate 22. Electrically conductive features may be formedby any process known in the art such as photolithography.

The following methods are useful for producing the composite materialdescribed above. It is recognized that variants of each method may beutilized and that different aspects of the various methods may be mixedtogether to produce a desired result. All methods generally requireappropriate degreasing or cleaning as a first step and rinsing, such aswith deionized water, between appropriate steps.

In accordance with a first embodiment, a carrier strip formed fromcopper or a copper alloy has a thickness effective to support a metalfoil layer. An exemplary nominal thickness for the carrier strip isapproximately 35-70 microns. The carrier strip is immersed in a diluteaqueous, alkaline sodium dichromate solution having the parametersspecified in Table 1. All solutions disclosed herein are aqueous, unlessotherwise specified. When a single value is given, that value isnominal.

TABLE 1 Sodium hydroxide 10-50 grams per liter (g/l) (broad range) 20-35g/l (preferred range) Chromium ions, such as from 0.1-10 g/l (broadrange) sodium dichromate 0.5-5 g/l (preferred range) OperatingTemperature 35° C.-50° C. PH Greater than 11 Counter Electrode Stainlesssteel Voltage 1 volt-6 volts Current Density 0.5-10 amps per square footAnodic step (ASF) (broad range) 1-5 ASF (preferred range) CurrentDensity 0.5-40 amps per square foot Cathodic step (ASF) (broad range)1-20 ASF (preferred range) Time (anodic step) 1-60 seconds (broad range)5-20 seconds (preferred) Time (cathodic step) 1-60 seconds (broad range)5-20 seconds (preferred)

The carrier strip is immersed into an electrolytic cell containing theelectrolyte and a voltage is impressed across the cell with the carrierstrip as the anode. The anodic treatment generates a uniformmicroroughness on the surface of the carrier strip and induces asubsequent uniform metal foil layer copper deposit. On completion of theanodic treatment, the carrier strip is maintained in the sameelectrolyte and the polarity of the electrolytic cell is reversed. Thecarrier strip is made the cathode to deposit a thin, on the order of10-500 angstrom, layer that is believed to be an admixture of chromiumand chromium oxides on the carrier strip. This admixture forms therelease layer that facilitates separation of the carrier stripsubsequent to lamination or other processing.

The release layer is formed to a maximum thickness of about 500angstroms. When the release layer thickness exceeds this maximum, theminimum release force requirements are not consistently achieved. Sincethe thickness of the release layer may be less than the microscopicsurface roughness of the copper foil, the precursor anodic treatment isused to achieve a more uniform surface finish.

Subsequent to rinsing, a seed layer of copper with a nominal thicknessof between 0.1 and 0.5 micron of copper is deposited on the releaselayer utilizing the parameters specified in Table 2.

TABLE 2 Copper ions, such as from 5-35 g/l (broad range) copper sulfateand/or 15-25 g/l (preferred copper pyrophosphate range) Optionalinclusions of Amount as required leveling agents, complexing agents andsurfactants Operating Temperature 35° C.-70° C. pH 6-10 Anode MaterialStainless Steel or copper Voltage 3-7 volts Current Density 10-50 ASFTime 5-100 seconds

The seed layer forms a nucleating agent for the subsequent high speeddeposition of a copper foil layer. While the seed layer is preferablyformed from copper, it may be any electrically conductive metal that canbe deposited in a mildly acidic to alkaline solution and is etchable inthe same chemical solutions as copper. Such other metals for the seedlayer include copper alloys, tin, iron, zinc, nickel, cobalt, etc. Theseed layer protects the release layer from chemical attack in anelectrolyte utilized to deposit the bulk of the metal foil layerthickness. Typically, to maximize manufacturing speed, an acid copperelectrolyte as specified in Table 3 is utilized.

Alternatively, immersion time in the copper containing alkalineelectrolyte, such as that disclosed in Table 2, is increased to on theorder of 100 seconds to 20 minutes to deposit a copper layer with athickness of from approximately 1.0 to 15 microns. In this aspect of theinvention, the subsequent step of building up the copper thickness in acopper containing acidic electrolyte is omitted.

TABLE 3 Copper ions, such as from 20-80 g/l (broad) copper sulfate 50-70g/l (preferred) Sulfuric acid 30-200 g/l (broad) 40-100 g/l (preferred)Operating Temperature 25° C.-70° C. PH Less than 1.5 Anode Material Leador copper Voltage 5-10 volts Current Density 30-1000 ASF (broad) 40-500ASF (preferred) Time 0.5-8 minutes (broad) 1-5 minutes (preferred)

To enhance adhesion, a dendritic treatment may be used to roughen theoutside surface of the metal foil layer. One suitable dendritictreatment utilizes the parameters specified in Table 4. Alternatively,an anti-tarnish layer such as a mixture of chromium and zinc may bedeposited to increase adhesion without increasing surface roughness.

TABLE 4 Copper ions, such as from 15-70 g/l (broad) copper sulfate 18-25g/l (preferred) Sulfuric acid 10-200 g/l (broad) 35-100 g/l (preferred)Sodium lauryl sulfate 1-20 ppm Operating Temperature 25° C.-55° C. pHLess than 1.5 Anode Material Lead or copper Voltage 5-10 volts CurrentDensity 50-1000 ASF (broad) 100-500 ASF (preferred) Time 4-60 seconds(broad) 4-40 seconds (preferred)

In a second embodiment, a carrier strip as described above is immersedin the solution of Table 1 for a time of from two to sixty secondswithout utilizing electric current. A nominal 5 micron copper foil layerand dendritic treatment is then applied as above.

In accordance with a third embodiment of the invention, a copper carrierstrip, as described above, is electrolytically coated with a thin, onthe order of between 0.05 micron and 2 microns, layer of nickelutilizing the parameters described in Table 5.

TABLE 5 Nickel sulfamate 150-600 g/l (broad) 400-500 g/l (preferred)Nickel chloride 0-15 g/l (broad) 0 7 g/l (preferred) Boric acid 25-50g/l (broad) 35-45 g/l (preferred) Operating Temperature 45° C.-60° C. pH2-5 Anode Material Nickel or Stainless Steel Voltage 0.5 5 volts CurrentDensity 20-60 ASF Time 20-60 seconds

A chromium phosphate release layer is then applied over the thin layerof nickel by immersion in a dilute chromic acid/phosphoric acid solutionhaving the parameters disclosed in Table 6.

TABLE 6 Chromic acid 0.1-20 g/l (broad) 0.2-10 g/l (preferred)Phosphoric acid 0.1-80 g/l (broad) 0.5-40 g/l (preferred) OperatingTemperature 20° C.-60° C. pH 0.1-3 Time 5-120 seconds (broad) 10-40seconds (preferred)

A nominal 5 micron copper foil metal layer is then deposited as abovefollowed by a dendritic treatment as above.

In accordance with a fourth embodiment of the invention, a thin, on theorder of between 0.05 micron and 2 microns, layer of nickel is depositedon a copper alloy carrier strip as above. A release layer is depositedfrom an aqueous solution containing sodium hydroxide as disclosed inTable 7.

TABLE 7 Sodium hydroxide 10-80 g/l (broad) 20-50 g/l (preferred)Operating Temperature 35° C.-60° C. pH Greater than 11 Counter ElectrodeStainless steel Voltage 0.5-5 volts Current Density (anodic 1.0-50 ASF(broad) step) 2.5-35 ASF (preferred) Current Density (cathodic 0.5-40ASF (broad) step) 1.0-25 ASF (preferred) Time (anodic step) 2-60 seconds5-30 seconds Time (cathodic step) 2-60 seconds 5-30 seconds

The nickel coated carrier strip is first made anodic and then cathodicto form reduced nickel oxides. Approximately 5 microns of copper is thenapplied as the metal foil layer followed by a dendritic treatment asdescribed above.

In each of embodiments 1-4, an alkaline copper plating bath waspreferably used to deposit a seed layer having a thickness of from about0.1 to about 0.5 micron of copper prior to depositing up to 5 microns ofcopper plating in an acidic bath. In the alternative first embodiment,the subsequent deposition of copper from an acidic bath is omitted. Theinitial use of an alkaline copper bath avoids potential attack to thechromium oxide, nickel oxide or nickel phosphate release layer as couldhappen in the acidic copper bath thus improving thereliability/integrity of the release interface. Embodiments five and sixdescribe methods for forming a composite material having similarreliability and integrity without the need for an alkaline copper bath.

In embodiment five, a smooth nickel deposit is formed on the copperalloy carrier strip utilizing a suitable nickel plating bath, such asthe nickel sulfamate electrolyte of Table 5. The nickel plated carrierstrip is then immersed in an aqueous electrolyte containing sodiumhydroxide utilizing the parameters recited in Table 7. The carrier stripis first made anodic and then cathodic. A copper metal foil layer isthen deposited using a copper sulphate bath (Table 3) followed bydendritic treatment (Table 4).

In a sixth embodiment, a thin layer of nickel, having a thickness on theorder of between 0.05 micron and 2 microns, is applied to the carrierstrip (Table 5) as described above followed by treatment in an aqueoussolution containing sodium hydroxide with the carrier strip firstforming the anode and then the cathode as in Table 7. Next, the nickelis treated cathodically in an acid copper sulfate bath at low currentdensity and the parameters illustrated in Table 8.

TABLE 8 Copper ions, such as from 40-80 g/l (broad) copper sulfate 60-70g/l (preferred) Sulfuric acid 50-100 g/l (broad) 60-75 g/l (preferred)Operating Temperature 35° C.-60° C. pH Less than 1 Cathode Material Leador copper Voltage 5-8 volts Current Density 0.03-2 ASF (broad) 0.05 0.5ASF (preferred) Time 30-120 seconds (broad) 45-90 seconds (preferred)

Copper deposition as in Table 3 is then utilized to increase thethickness up to a nominal 5 microns. Dendritic treatment as in Table 4completes the process.

Composite materials formed from any one of the above processes may thenbe used to manufacture either printed circuit boards or flex circuits asdescribed above. The advantages of the invention will become moreapparent from the examples that follow.

In an alternative embodiment of the invention, the presence of pinholesin the foil may be reduced by employing methods of surface preparationon the exposed surface of the carrier strip that result in a smoother,more uniform surface. Examples of such methods of preparation includemaking use of oxygen free copper for the carrier strip, as well assmooth rolling, micro-etching, and flashing the surface of the carrierstrip with copper or nickel solutions.

The presence of oxygen in copper carrier strips can give rise theformation of pits in the exposed surface of the carrier strip arisingfrom the presence of copper oxide particles. When the release layer andfoil are deposited over these pits, they conform to the structure of thepits. As a result, when the carrier strip is removed from the foil, theportions of the foil deposited into the pits often times break off fromthe foil. This breakage can result in pinholes on the surface of thefoil or undesirable imperfections to the otherwise relatively uniformsurface of the foil.

Therefore, one embodiment of the present invention incorporates the useof oxygen free copper for the carrier strip. The use of oxygen freecopper reduces or largely eliminates the presence of copper oxideparticles, thus reducing the attendant pits, and providing for a moreuniform foil surface both before and after separation from the carrierstrip.

Additional methods employed in the present invention to produce a moreuniform carrier strip surface include smooth rolling, microetching, andexposing the surface of the carrier strip onto which the release layeris to be deposited to a copper or nickel flash. Examples of microetchingcapable of smoothing the surface of the carrier strip to a desirableuniformity include, but are not limited to, the application of a 1lb./gal. ammonium persulfate solution with 3 v % sulfuric acid mixtureat 115° F. for approximately 50 seconds. As stated above, the carrierstrip may be coated with a copper or nickel flash to cover up defects,such as those incurred during rolling, that may adversely impact theuniform roughness of the surface of the carrier strip.

In another embodiment of the present invention, a dark andnon-reflective layer is interposed between the release layer and thefoil. The dark and non-reflective coating remains bonded to the foilafter separation from the carrier strip and enhances the ability of thecopper foil to be fashioned through the use of a laser.

There are, in general, two types of lasers that are used to drillthrough or otherwise remove portions of metallic foil. The first typeincludes CO₂ lasers which emit light in the IR range. The second type oflasers includes YAK lasers which emit light in the UV range. While YAKlasers can drill directly through copper foil, such lasers tend to do soat a relatively slow rate. In contrast, CO₂ lasers are capable oftransmitting more energy per unit of time to the foil and, hence,drilling through the foil at a relatively faster rate. However, CO₂layers are generally incapable of drilling through copper foil directly.Rather, in order to drill through metallic foil, a CO₂ laser requiresthat the surface of the foil be treated in such a manner as to render itboth dark and non-reflective. As used herein, non-reflective refers tothe property exhibited by a surface that substantially absorbs, ratherthan reflects, the energy contained in light which interacts with thesurface.

Therefore, with reference to FIG. 15, in one embodiment of the presentinvention, there is interposed between the release layer 16 and themetal foil layer 14 a dark layer 13 comprised of dark material. Afterthe carrier layer 12 and release layer 16 are removed, the dark layer 13remains bonded to the metal foil layer 14 to facilitate drilling by aCO₂ laser. The dark layer 13 is preferably formed of betweenapproximately 0.05 and 0.5 microns of a nickel-copper combination,cobalt, tin, manganese, iron, or nickel layer. Most preferably the darklayer is approximately 0.2 microns in thickness. In order to render thedark layer 13 non-reflective, the surface of the carrier strip 12opposite the dark layer 13 is made uniformly rough. Because the surfacetexture of the carrier strip 12 is imprinted upon the side of the darklayer opposite the attached foil, the dark layer assumes a surfacetexture similar to that of the uniformly rough carrier strip 12 surface.

There exist several methods by which the surface of the carrier strip 12can be made suitably uniformly rough. These methods include, but are notlimited to, nodule plating the surface of the carrier strip with pink orblack copper bonding such as CopperBond (a registered trademark of theOlin Corporation of Norwalk, Conn.), sandblasting the surface of thecarrier strip, microetching the surface of the carrier strip, and roughrolling the carrier strip.

The resultant non-reflective dark layer facilitates the use of a laser,preferably a CO₂ laser, to drill through the dark layer and theunderlying foil layer. In addition to drilling holes in this manner, thelaser may be manipulated to etch into the foil a desired circuit schemasuch as one required to form an integrated circuit.

EXAMPLES Example 1

A 2 oz. wrought copper foil was used as a carrier strip. The strip waselectrocleaned in an alkaline commercial cleaner using 20 ASF currentdensity for 40 sec. The foil was rinsed and then the release layertreatment was conducted in 20-35 g/l NaOH+0.5-5 g/l chromium ions assodium dichromate solution using an anodic current of 1-5 ASF followedby a cathodic current of 1-20 ASF for 5-20 sec. The anodic treatmentappeared to generate a uniform micro-roughness on the surface of thefoil and induce a uniform copper deposit. The cathodic treatmentappeared to deposit a transparent layer of chromium and chromium oxides,which is believed to be responsible for the release of the carrier stripafter lamination.

A seed layer of 0.1-0.5 micron copper was electroplated in an alkalinecopper plating solution. A 5 micron copper deposit was thenelectroplated, using 60-70 g/l copper ions as copper sulfate and 60-75g/l sulfuric acid at 40-100 ASF for 5.4-2.1 minutes, followed by adendritic copper or copper/nickel treatment. After lamination to an FR-4epoxy substrate, the 2 oz carrier was easily peeled with a measured bondstrength of 0.1-1.0 lb/in.

Example 2

A 2 oz. wrought copper foil was used as a carrier strip. The strip waselectrocleaned in an alkaline commercial cleaner using 20 ASF currentdensity for 40 sec. The foil was rinsed and then the release layertreatment applied by electroplating in a solution of 20-35 g/lNaOH+0.5-5 g/l chromium ions as sodium dichromate. This treatmentappeared to form a transparent layer of chromium and chromium oxides.

A seed layer of 0.1-0.5 micron copper was electroplated in an alkalinecopper plating solution. A 5 micron copper deposit was thenelectroplated using 60-70 g/l copper ions as copper sulfate and 60-75g/l sulfuric acid at 40-100 ASF for 5.4-2.1 minutes, followed by adendritic copper or copper/nickel treatment. After lamination to an FR-4epoxy substrate, the 2 oz. carrier was easily peeled with a measuredbond strength of 0.1-1.0 lb/in.

Example 3

After cleaning the copper carrier strip, a nickel layer was firstelectroplated with 0.15 micron nickel in a nickel sulfamate bath at 30ASF for 20 sec. The foil was then immersed in a solution containing0.2-10.0 g/l chromic acid and 0.5-40 g/l phosphoric acid for 10-40 secat ambient temperature. The alkaline copper seed layer and acidic copperplating were conducted as described in Example 1. A peelable foilresulted after lamination with 0.2-2.0 lb/in release force.

Example 4

As in Example 3, a nickel layer was first electroplated. The nickelsurface was then anodically treated in a 20-50 g/l NaOH solution at0.5-10 ASF for 5-30 sec to generate a nickel oxide release layer. Thisnickel oxide layer was then cathodically reduced in a 20-50 g/l NaOHsolution at 0.5-50 ASF for 5-30 sec. This cathodic treatment appeared toproduce reduced oxides and enlarge the operating window. Without thecathodic treatment, if the anodic current is too low, a non-peelablefoil would be produced. If the anodic current is too high, the plated 5micron foil often delaminates or forms blister even before laminationand renders the product useless.

After the nickel and nickel oxide treatment, the alkaline copper seedlayer and 5 micron acidic copper are deposited. A release force of0.35-1.0 lb/in was obtained.

Example 5

The formation of a smooth nickel deposit onto a carrier strip has beenshown to be very easily accomplished by deposition out of a nickelsulfamate bath using a current density of 30 to 50 ASF with times of 20to 30 seconds.

Treatment of the nickel plate in a solution of 30 g/l sodium hydroxidewas then conducted with an anodic treatment of 20 to 40 ASF for 20 to 40seconds with subsequent cathodic treatment in the same solution at 25%to 50% of the anodic current density and for half the time. No seedlayer was used, rather the nickel coated support layer was cathodicallytreated in acid-copper sulfate bath at low current density, 60 secondsat 0.03 to 2 ASF, followed by plating of copper from the acid coppersulfate bath at 65 ASF for 3.5 minutes to achieve the desired 5 micronthickness. The foils retained their peelability following lamination.

For comparison, it was demonstrated that the above low current treatmentresulted in peelable foils free of defects while foils made usingidentical conditions, but without the low current treatment, were fullof defects and not peelable following lamination.

Example 6

The effect of the release layer on the separation force required toseparate a copper foil layer from a copper support layer is graphicallyillustrated in FIG. 14. Both a chromium metal release layer, as knownfrom the prior art, and a chromium plus chromium oxide release layer asdisclosed herein were effective to provide a relatively low, less than8.9 kg/m (0.5 pounds per inch), separation force at room temperature.Even when heated to temperatures of up to 200° C. and remaining attemperature for one hour, the separation forces are about equal.However, at temperatures above 200° C., the separation force for thechromium release layer (reference line 38) increases rapidly while thereis no increase in the separation force for the chromium plus chromiumoxide release layer (reference line 40).

This example illustrates the wider processing window available forlamination utilizing the release layer of the invention.

Example 7

FIGS. 8 and 10 illustrate that more vertical side walls 42 are obtainedutilizing the 5 micron foil as compared to the 12 micron foil. To avoidthe presence of short circuits, etching must be for a time effective toensure substantial removal of copper from the surface of the dielectric22. Since the surface of the foil opposite the dielectric carrier 22 isexposed to the etching solution for a longer time than the surfaceadjacent the dielectric carrier, a circuit trace width reduction at thatopposing surface occurs. Due to the longer etching time for the 12micron foil, this over-etching is more pronounced.

Table 9 demonstrates conductive circuit traces having a nominal width asindicated when formed by photopatterning a 38 micron thick dry filmphoto resist and then etching in an alkaline solution at a temperatureof 52° C. The 5 micron foil required approximately half the etch time ofthe 12 micron foil. From Table 9, it may be seen that the conductorwidth is closer to that of the designated conductor width for 5 micronfoil than for 12 micron foil.

TABLE 9 Etched Conductor Designed Conductor Width Width (μm) (μm) 5 μmfoil 12 μm foil 50 40 24 75 68 52 100 92 76 125 117 101

TABLE 10 % Fewer Defects Feature (5 μm vs. 12 μm foil)  50 μm conductors30  75 μm conductors 4 100 μm conductors 65 125 μm conductors 62  50 μmspaces 31  75 μm spaces 19

TABLE 11 After After lamination lamination Sample Anodic/Cathodic C7025C110 ID Current (asf) carrier carrier 1 1.7 Cracked Weak open release 21.25 Weak Weak release release 3 1.1 Cracked Weak open release 4 1.0Weak No release release 5 0.8 Cracked No open release 6 0.6 No releaseNo release Additional Parameters: Anodic current plating time = 10seconds Anodic/cathodic bath temperature = 36-37 C. Alkaline coppercurrent 30 asf × 20 sec, 52-55 C.

A 1 ounce C110 foil and 1 ounce C7025 foil were utilized to form carrierstrips. Prior to cleaning in NaOH the foil samples were immersed in 10%sulfuric acid to remove any surface oxides. To provide consistency noneof the solutions were stirred. As the objective of the test was tocompare the capability of foils of differing tensile strengths inproviding a controllable release force, the parameters under which thetests were conducted were held constant while the two foils differedonly in tensile strength. The tensile strength of C7025 wasapproximately 100 ksi after lamination and the tensile strength of C110was approximately 30 ksi after lamination.

As table 11 illustrates, the release force of both foils is dependentupon the anodic/cathodic current parameters. The break off point foranodic/cathodic current is 0.8 asf for the C7025 foil and 1.1 for theC110 foil. As used herein, “break off point” refers to the minimumanodic/cathodic current treatment value at which release is enabled.

It is apparent that there has been provided in accordance with thepresent invention a composite material including a releasable metal foillayer and methods for the manufacture of such a composite material thatfully satisfies the objects, means and advantages as set forth hereinabove. While the invention has been described in combination withembodiments thereof, it is evident that many alternatives, modificationsand variations will be apparent to those skilled in the art in light ofthe foregoing description. Accordingly, it is intended to embrace allsuch alternatives, modifications and variations as fall within thespirit and broad scope of the appended claims.

1. A composite material, comprising: a carrier strip said carrier stripcomprising a first side said first side possessing a substantiallyuniform roughness wherein said substantially uniform roughness resultsfrom flashing said first side of said carrier strip with copper ornickel; an electrolytically deposited copper foil layer having opposingfirst and second sides and a thickness of from 0.1 micron to 15 microns;a dark layer effective to absorb light energy said dark layer havingopposing first and second sides said first side of said dark layer incontact with said second side of said copper foil layer; wherein saiddark layer has a thickness between 0.2 and 0.5 microns, said dark layercomprised of a material selected from the group consisting of copper,nickel, tin, manganese, iron, and copper-nickel alloys; and a releaselayer effective to facilitate separation of said carrier strip from saiddark layer disposed between and contacting both said first side of saidcarrier strip and said second side of said dark layer and effective totransmit the surface characteristics of said first side of said carrierstrip to said second side of said dark layer.